Stress and Strain in Geology

Introduction

• Stress and strain are terms with specific meanings in science, particularly in the mechanics of materials.
• Tectonic forces cause deformation, with stress being a force with magnitude and direction.

Types of Stress

• Compression: Squeezing stress.
• Tension: Stretching or pulling apart stress.
• Shearing: Sliding stress.

Plate Tectonics and Stress

• Three main types of plate boundaries:
  • Plates coming together (Compression)
  • Plates pulling apart (Tension)
  • Plates sliding past each other (Shearing)
• These motions cause deformation in rocks.

Strain: Response to Stress

• What happens to a rock when it's under stress.
• Examples from rock deformation lab:
  • Undeformed cylinder.
  • Brittle behavior: Rock cracking.
  • Ductile behavior: Rock flowing.

Rock Behavior Under Stress

• Ductile Behavior: Rocks flow under high pressure and temperature, similar to silly putty.
• Brittle Behavior: Rocks crack or break.

Types of Deformation

• Six potential end members based on stress type and rock behavior:
  1. Compression (Ductile)
  2. Compression (Brittle)
  3. Tension (Ductile)
  4. Tension (Brittle)
  5. Shearing (Ductile)
  6. Shearing (Brittle)

Practical Exercise

• Create a chart for ductile and brittle behavior under different stresses.
• Consider compressive, tensional, and shearing features:
  • Compression:
    • Ductile: Folding
    • Brittle: Fault formation with one side pushed up
  • Tension:
    • Ductile: Stretching and thinning
    • Brittle: Faulting with stretching apart and dropping down
  • Shearing:
    • Ductile: Bending in horizontal plane
    • Brittle: Strike-slip fault formation

Conclusion

• Understanding stress and strain is crucial for analyzing geological features and behaviors.